The present invention relates to a colour display tube having, in an envelope, means for producing at least one electron beam, a target including a target electrode and bands of parallel phosphor lines. The phosphor lines of each band luminesce in different colours. A mesh electrode mounted adjacent to the target and a planar electrode extending substantially parallel to the mesh electrode define a trajectory control space. Means is provided for introducing each electron beam along an inclined path into the trajectory control space where the beam follows a parabolic trajectory and approaches the target at a substantially constant angle. In a particular application, the present invention relates to a compact colour display tube of small dimensions, such as one having a screen of approximately 12.5 cm (5 inches) diagonal.
A compact monochrome display tube is disclosed in British Patent Specification No. 865,667.
In this patent specification embodiments are disclosed in which an electron beam is produced by an electron gun whose axis is inclined or is parallel to the plane containing a fluorescent screen. At a predetermined point in the trajectory of the electron beam it enters a field produced by two electrodes at different potentials. One of the electrodes is connected to the fluorescent screen. The beam which enters the field at a constant angle .beta. follows a parabolic path to impinge upon the fluorescent screen at a certain angle .alpha. independently of the distance traversed. The trajectory is determined by applying a signal at the field time base frequency to the field producing electrode opposite the screen. By means of deflection electrodes the electron beam can be scanned at line frequencies and a raster scan can be achieved. Since the electron beam lands on the fluorescent screen at the same angle for all parabolic trajectories, the spot of the electron beam on the screen will always have substantially the same shape.
A colour display tube of the kind mentioned in the opening paragraph is disclosed in British Patent Specification No. 1,223,723. In that patent specification an orthogonally focused, single electron beam passes through a frame scanning means and is deflected through 225.degree. in order that it can enter a trajectory control space at an angle of 45.degree.. The trajectory control space is defined by a planar reflecting or repelling electrode and a woven mesh screen grid between which electrode and grid a sawtooth voltage having a line scanning frequency is applied. The beam is then decelerated by a colour selection electrode of the Venetian blind type, the potential of which colour selection electrode is modulated at a high frequency, produced for example by mixing the colour subcarrier and its first harmonic. This is done in such manner that the electron beam, as it line scans across a screen on the side of the Venetian blind electrode remote from the screen grid, is deflected onto a particular phosphor line of a band of at least two parallel phosphor lines. The provision of a screen grid and a Venetian blind type electrode having a large number of louvres makes the construction of the tube complicated, costly and unsuitable for use with compact display tubes having screens of the order of 12.5 cm. diagonal. A Venetian blind type of electrode for such a small size of display tube would be fragile and difficult to make because not only would the spacing between adjacent louvres have to be substantially less than 1 mm. to avoid losing resolution but also each louvre has to be set at exactly the same angle.